Near-infrared absorptive dyes are used for various purposes in wide fields. The dyes are used in, for example, infrared-cutting films for plasma display panels (PDPs) and solid-state image sensing devices such as CCDs, optical filters in heat-shielding films, or photothermal materials in write once optical disks (CD-Rs) or flash-meltable and fixable materials.
Moreover, the near-infrared absorptive dyes are used as information displaying materials for security inks or invisible bar code inks. The security inks are utilized to print encrypted data (such as a bar code, a two-dimensional code, or OCR characters) on paper moneys, vouchers, valuable securities, or the like, in order to prevent forgery. The inks are further used as hidden inks, which do not damage the design of a printed material.
When a near-infrared absorptive dye is employed for information display applications, it is very important for the dye to have very strong absorption in the near-infrared region and good non-visibility which indicates that it is not visible to naked eye. It has been suggested to form an infrared absorptive image by using an ink containing inorganic ion(s) (copper, iron and ytterbium, etc.), or an organic dye such as phthalocyanine dye, dithiol compound dye, squarylium dye, chroconium dye, and nickel complex dye, etc. (see, JP-A-8-143853 (“JP-A” means unexamined published Japanese patent application), JP-A-7-164729, JP-A-11-279465, JP-A-2008-291072 and JP-A-2002-146254). Although the infrared absorption property of the image created by using these inks is sufficient, it cannot be said that these inks have sufficient non-visibility. When trying to accelerate the non-visibility, the infrared absorption property becomes insufficient, and as a result, an ink having poor discrimination ability as it is mixed with a visible image is obtained. In particular, with respect to the near-infrared absorptive dyes having maximum absorption in an infrared range close to a visible range (i.e., 700 nm to 900 nm), tendency of showing insufficient non-visibility is very obvious.
As another application example of the near-infrared absorptive dye, there is a near-infrared absorptive filter for CCDs. The solid-state imaging sensing device such as CMOSs and CCDs, which are installed in a camera, has high sensitivity even for the rays of the infrared range (700 nm to 1100 nm) so that accurate color resolution cannot be obtained. To avoid such inaccurate color resolution, a reflection-type infrared absorptive filter and an absorption-type infrared absorptive filter using an inorganic ion or an organic dye are generally set in an optical system of a camera. However, since these filters are applied over the entire surface of a lens, turning ON and OFF of an infrared cut for each pixel is practically impossible. Thus it cannot be used, for example, for an application in which both an image by visual light and an image by infrared light are simultaneously taken.